I've chosen to execute one of Earl Stahl's free flight rubber plans. This gentleman created many wonderful models over the course of his life and it is a real loss to the modeling community when he passed away recently.

I have chosen to model the Blackburn Skua, a Royal Navy Dive Bomber dating to early WW2. It was presented as a 25 inch wingspan rubber free flight model in the June 1942 issue of model airplane news.

Eight of the Fifteen Skuas from 800 and 803 Squadron,were shot down during their attack on the German battle cruisers Scharnhorst and Gneisenau in Trondheim harbour. Two of them were shot down by Messerschmitts here. Skua 7L:L2992 crashed into the hillside above the farm at Kjøra. The two pilots were brought to Orkdal hospital, but Sub Lieutenant John Anthony Harris died just after arrival. Naval Airman Stuart Rex Douglas Stevenson died at the hospital a year later 31.5 1941

Skua 7A (probably L2896) ditched into the Orkdalsfjord at Kjøra. Lieutenant Peter Evelyn Fanshawe and Lt/Cdr. John Casson were rescued by locals and spent the rest of the war as PoW at Stalag Luft III. They were shot down by Ofw.E. Sawallisch of II./JG 77.

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On April 25th. 2007, another Skua were found. This was Skua 7A from 803 Sqadron - Pilot: Lt. Cdr. J. Casson RN, and Observer Lt. P. E. Fanshawe, RN. It was shot down during the attack on Scharnhorst, on June 13th. 1940, known as Black Thursday. The Skua ditched at Geitastand, nearby Trondheim. The aircraft was found by ROV from R/V Gunnerus, NTNU students, Torkell Nodland, and Klas Gjolmesli. The Skua was standing upright on the bottom, with minimal damage at a depth of 242 meters.

I'm beginning to consider the transformation of the plan from rubber free flight to radio control. I will consider the aircraft from the point of view of a rubber free flight model.

Preflight: the aircraft is designed to accommodate a fairly long rubber motor. The fuselage is designed as a compression cage, with four large stringers, two on Buttline 0, and two on the water line coincident with the center of the spinner. Smaller stringers and some strength to the compression cage but mainly serve to support the outer surface the aircraft.

Launch: the aircraft would probably rise off ground, with a smooth runway. The landing gear or oriented such that the main strut is vertical in relation to the aircraft ground resting angle. As the aircraft tail rices off the ground, the sooner gravity would rotate around the main gear axle and make it very prone to tipping over. I assess the aircraft was designed for hand launch.

Flight: the aircraft is essentially a powered glider it would be trimmed to climb slightly under power in the transition to a best glide condition once the rubber motor was exhausted. The wing structure is such that it probably would not withstand more than one and a half or two times gravity. The spar is a simple 1/16 inch sheet.

Landing: the landing gear is designed to transmit forces back into the wing spreading them over a large area, and to resist bending.

Considerations for radio control

The aircraft, under radio control, will have several fixed masses to replace the rubber motor: the motor, three servos (as I intend to add ailerons), the receiver, speed controller, and battery. The long nose moment gives me some latitude in the placement.

The motor of course will move forward and frame Bravo will probably become a firewall. I'm leaning towards making frame bravo a split, allowing removal of the cowling, which would be attached by magnets.

In the event of a less than perfect landing, the other masses in the aircraft, located in the vicinity of the center of gravity, would tend to move forward. This would put the aircraft structure between frame Bravo and frame Delta under some significant compression. Therefore, I intend to apply some sheeting from Bravo to Delta, probably 1/32 balsa sheet. I will probably make the main stringers in the forward portion of the aircraft out of Basswood; and may continue the main stringers and Basswood through the entire fuselage.

I will need a tray for the electronics above the wing and will probably make the canopy a removable hatch. Since the rear cockpit enclosure is optional, it makes a convenient exit for cooling air from the fuselage which I can bring in through the cowling.

Back to the wing. I'm going to use strip ailerons on the aft edge of the wing. This will require an extra sub spar, and slight modifications to the wing ribs. Since I intend to fly the aircraft in a semi-scale manner, I suspect I need to beef up the wing slightly. The way the wing is designed now spars will handle some tension is the wing loading goes up the lack of a upper spar means the wing will flex quite a bit more than I feel comfortable accepting.

So I will likely build the wing almost as designed but, I will probably add two small graphite rods to the upper surface of the wing, one above the spar and one about halfway between the spar and the leading edge. I will probably add a third to the underside of the main spar. These can all be one millimeter diameter and still meet my desire of making the wing stronger in loading and stiffer and torsion. After I build the wing I will see how soft and torsion it is and perhaps add another graphite rod to the trailing edge.

I should be able to build the horizontal and vertical stabilizer with minimum changes, adding a second main spar adjacent to and parallel to the first selecting split the flying surfaces off and hinge them. I'll need to add a small gusset to accommodate the control horns, and will need to connect the two halves of the horizontal stabilizer.

As 95% of my flying is from grass fields, I will build the airplane in the wheels up condition and probably add a wire skid that exits about frame bravo and curves down and back along the aircraft centerline. This skid really reduces any tendency the aircraft to plow under the ground and prevents a good deal of grass stains and dings during landing.

I will hold off any judgment about motor until I get somewhat of an idea of the flying weight.

At 30" you should aim at about 200 grams for a free flight conversion. Until about 240 she'll fly.

Regarding the wing: i think you could greatly improve stability by just adding 2 or 3 Balsa stringers on top of the wing in the first third of the chord. I've built a couple of Herr free flight conversions and they all used wings like that which provide enough stability for aerobatic flight.

Interesting subject. I've built Earl Stahl's Fairchild 24 at 36" and his Hurricane at 30", and both were nice, well-engineered models. If I were you, I'd use a completely different spar configuration, opting for two 3/32" square bass spars, one at the top of the wing and the second along the bottom, joined between the ribs with 1/32" balsa shear webs. Your conversion will be around 3 times the flying weight of the original, and so the spar needs more strength.

Thanks for the insight, Steve. I was worrying over wing structure today, as my Balsa USA box arrived yesterday.

I was thinking I'd leave the original "half-web" star and add a top spar, as you did. But, now that I see what you are talking about, I believe I will follow suit. I also like the way the added the secondary spars for torsion control.

No worries, David. The stiffest spar configuration would be an "I" beam, with the shear webs fitted between the upper and lower square spars, but it's pretty tedious to build. If you glue the shear webbing to the front of both spars, you end up with a "C" beam, which is still very strong while being much easier to build. You probably know this already, but the shear webs' grain should be vertical rather than horizontal, since their function is to keep the upper and lower spars from flexing.

But your intro to this build thread is captivating Dave.
We have a thread over in Australia on warbird finds and I posted your post to the Norwegian Skuas therehttp://www.rcgroups.com/forums/showthread.php?t=1693108
I can't recall anyone doing a Blackburn model, let alone a Skua so am keen to see your work.
Good luck and happy landings for your entry

Thanks, Phil. I'll certainly try to do the Skua justice. My intent is that my effort will produce a light sport scale model, as opposed to a very light, radio assisted, electric free flight model. I'll be setting up my work area tonight.